The use of fiber-optic communication systems for transmitting information by sending light signals through an optical fiber is omnipresent in the telecommunications industry. Optical fibers are used to transmit various kinds of communication signals over long distances via a vast amount of existing interurban and submarine transoceanic fiber communication lines.
In order to ensure the undisturbed detection of the communication signals after propagation over long distances, discrete optical amplifiers are deployed along the optical transmission line in multiple locations to energetically boost the signal being transmitted. These optical amplifiers are at amplification locations which can be spaced hundreds of kilometers apart, typically between 50 km and 120 km, and may be placed on remote locations, possibly under the sea.
Distributed optical amplification is a technology allowing to further increase performance that consists in exploiting the Raman gain induced by stimulated Raman scattering (SRS) in a given medium. The Raman-active medium is often the optical transmission fiber itself, which enables distributed amplification before the signal reaches the terminal site. In Raman amplification, a lower frequency “signal” photon induces the inelastic scattering of a higher frequency “pump” photon in the Raman-active medium. As a result of this inelastic scattering, another “signal” photon is produced, while the surplus energy is resonantly passed to the vibrational states of the medium. This process hence allows for an all-optical amplification.
In contrast to remote optically pumped amplifiers (ROPAs) that are for example known from US 2012/0033293 A1, distributed Raman amplification can be installed in an existing fiber link without modifying the fiber infrastructure, i.e. without having to access intermediate locations of the fiber. Thus, distributed Raman amplification is a convenient technology for upgrading existing fiber links at reasonable cost, since only the equipment at the terminal sites needs to be modified or replaced.
In ROPA-based systems, a ROPA cassette containing an erbium-doped fiber (EDF) is embedded into the fiber link at a remote location, typically between 80 km and 120 km away from the receiving site. Thus, signals propagating in the transmission fiber are amplified in a dedicated piece of fiber when their power levels are significantly higher than at the receiving site. The pump power is supplied to this remote ROPA cassette from the receiving site via the segment of the transmission fiber connecting the remote location with the receiving site. In this segment, the pump power propagating in opposite direction to a transmitted data optical signal that has already experienced amplification in the ROPA cassette will further amplify the data optical signal due to SRS. But the performance benefit mainly stems from the amplification in the ROPA cassette. Pump power provided to the ROPA cassette is almost completely absorbed in the ROPA cassette such that the pump power provided from the receiving site does not induce any noticeable Raman amplification in the segment of the transmission fiber between the ROPA cassette and the transmit site, at which the data optical signal is emitted.
Typically, fiber optic communication systems comprise an optical supervisory channel (OSC) on which information for monitoring and managing the functioning of the fiber optic communication system is transmitted. For example, the OSC may be employed to send instruction signals to different devices along the fiber optic communication system. Further, the OSC can be used to check fiber continuity and integrity. An interruption in the reception of the OSC may be used to detect a fiber breakage. Such fiber breakages constitute a severe hazard when using technologies employing high pumping powers, as is the case in distributed Raman amplification. When leaking from a broken fiber, the high power light used for Raman amplification poses a potential danger for the skin and eyes of a human operator supervising or manipulating the system or a passer-by. For this reason, laser safety mechanisms have been developed, which ensure that fiber integrity is checked and confirmed before and while high power light is launched into a fiber optic communication system. An OSC signal transmitted on said OSC is commonly used for this purpose.
Once the broken fiber has been repaired, such OSC signal can in principle be used to indicate fiber integrity to the Raman amplification device, such that the amplification, in particular the Raman pumping, can be resumed. However, the transmission of an OSC signal over long distances in a fiber optic communication system requires in long spans the use of optical amplification in the fiber as much as the transmission of any data optical signal does. Consequently, in absence of the Raman amplification, namely when normal operation of the Raman pumps has been interrupted upon detection of a fiber breakage as stipulated by laser safety measures, the OSC signal does not benefit from Raman amplification in the fiber either, and can therefore often not be transmitted over sufficiently long distances to indicate the restored fiber integrity. Accordingly, without ensuring sufficient strength of the transmitted OSC signal even in absence of the Raman amplification, the OSC signal cannot be used for triggering the resumption of the Raman amplification.
Different solutions have been proposed in order to overcome this problem described above. One of them, disclosed in US 2006/0140626 A1, refers to the possibility of arranging conventional optical amplifiers between a wavelength selective coupler separating the OSC from the data optical signal and an OSC receiver to directly amplify the OSC signal before it reaches the OSC receiver. However, this solution adds significant equipment costs and requires additional space for installation of the extra amplifier.
A further alternative not making use of such additional amplifiers relies on the use of an optical switch for directing pumping power designated for Raman amplification in the fiber into an amplifier in which the OSC signal can be amplified. However this has the drawback of leaving less pumping power available for Raman amplification in the fiber during normal operation due to the insertion losses of the switch. Further, optical switches able to withstand the corresponding high power levels are scarce and very costly.
In U.S. Pat. No. 8,228,598 B2, an optical amplification mechanism is disclosed that combines Raman amplification in discrete amplifiers with amplification in an erbium-doped fiber (EDF) by launching residual Raman pump power into an EDF. However, no method is disclosed therein for ensuring correct detection of an OSC signal when normal pumping conditions of the Raman pumps are prohibited for laser safety reasons.
In view of the above, there is a need for technical improvements in the OSC sensitivity of Raman pumping arrangements.